Experimental investigation of kerogen structure and heterogeneity during pyrolysis

Kerogen in shale microstructure plays a vital role in hydrocarbon generation, retention and accumulation. However, characterization of kerogen structure is challenging arguably due to a complex microporous structure and chemical heterogeneity. Thus, the role of kerogen molecular structure in controlling porosity and its heterogeneity, as well as its evolutionary processes, remains unclear. In this study, we investigate the chemical and pore structure of heated kerogen samples by employing a range of characterization tools including gas adsorption experiments, high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) spectroscopy, and laser Raman experiments for a broad range of pyrolysis temperatures (420–850 °C). Multifractal theory and peak fitting techniques were used to evaluate the controlling mechanisms of kerogen pore structure and heterogeneity. The results suggest that with increasing thermal maturity, the content of oxygen functional groups and long-chain aliphatic groups in kerogen significantly decreases, while the length of aromatic fringes increases and gradually becomes oriented. Furthermore, with increasing thermal maturity, the pore volume of kerogen exhibits a non-monotonic trend, i.e., a decrease, followed by an increase, and then finally a decrease. This is primarily influenced by processes such as hydrocarbon production weakening, secondary hydrocarbon cracking, and carbonization. The heterogeneity and dispersion of meso to macro-sized pores in kerogen increase with temperature, while the connectivity gradually decreases. The chemical structure and spatial distribution of kerogen significantly affect the heterogeneity, connectivity, and dispersion of meso to macro-sized pores. Specifically, high aromaticity, structural disorder, and short aliphatic chains result in enhanced heterogeneity and dispersion of meso to macro-sized pores, reducing pore connectivity. This study thus contributes to a deeper understanding of the evolution of organic matter pores.